1. Field of the Invention
The present invention relates to a method of polishing semiconductor wafers more particular to an effective technique suitable for polishing semiconductor wafers, whose surfaces to be polished are required to be very flat.
2. Description of the Related Art
The final step of manufacturing semiconductor silicon wafers includes a polishing step for forming a specular surface. This step generally employs a method called the mechanochemical method, which combines mechanical attrition and chemical reaction.
FIG. 4 shows the main components of a polishing apparatus for polishing one face of a semiconductor wafer. As shown in FIGS. 4 and 5, numeral 1 indicates a glass plate. A plurality of semiconductor wafers 2 are bonded with wax to the under surface of the glass plate 1. These semiconductor wafers 2, having undergone processes such as lapping, beveling and etching, are bonded in such a manner that they can be attached or removed. A polishing cloth 3a is firmly held on the surface of a turntable 3, which is positioned under the glass plate 1. Polishing is performed by using the apparatus in the following way. The semiconductor wafer 2 contacts the polishing cloth 3a under the pressure of the glass plate 1. At the same time, the turntable 3 rotates to cause the glass plate 1, supporting the semiconductor wafer 2, to rotate so as to bring the semiconductor wafer 2 into contact with the polishing cloth 3a on which polishing slurry is sprayed. As a result, the main surface of the semiconductor wafer 2 bonded to the underface of the glass plate 1 is polished. The polishing slurry is a weak alkaline aqueous solution containing colloidal silica as fine abrasive grains.
With an increasingly strong demand in recent years for high precision flatness in the semiconductor wafer surface to be polished because of microscopically fine patterns of semiconductor ICs, the following problems have arisen with the above-described polishing method.
When the semiconductor wafer is polished by the above-mentioned polishing apparatus, quality changes of the polishing cloth 3 occurs over time. In addition, deformation of the glass plate 1 caused by pressure applied when the semiconductor wafer 2 comes in contact with the polishing cloth 3a occurs, and different rotation speeds of the turntable 3 at various positions along the radius of the table 3 also take place. Uneven thickness of the polished semiconductor wafer caused by the above cited phenomena cannot be neglected from a view point of requirements for semiconductor IC devices's sophistication in recent years.
The uneven thickness gives much more influence on semiconductor on insulator (SOI)-structured devices having an extremely thin active zone.
The more the semiconductor wafer 2 is pressed against the polishing cloth 3a, the faster the polishing speed becomes. It is difficult, however, to control the polishing amount with a fast polishing speed. On the contrary, it is easy to control the polishing amount with a slow polishing speed, though polishing is time-consuming.